thanks all for the feedback. much appreciated.
i had no idea that those 2 wheels were no identical ( liv n nevr learn )
iŽll wait till the 2nd wheel i ordered gets in and then take the beast apart.
roughly speaking: the difference betw jockey and idler are... the bearings??
and: could i convert a new idler (which what i bought, i guess) with parts of the old worn-down jockey into a ŽnewŽ jockey?
anyhow, in case somebody is curious, thats the gear i bought (and that causes skipping):
ebay.es + /itm/Bicicleta-Jockey-rueda-de-aluminio-derailleur-trasero-11T-engranaje-Gu%C3%ADa-polea/223327827520?ssPageName=STRK%3AMEBIDX%3AIT&var=522118783341&_trksid=p2057872.m2749.l2649